1. Field of the Invention
The present invention relates to methods for forming subterranean barriers for purposes of containment, typically containment of solid and liquid waste. The techniques described herein are applicable to both vertical and horizontal barriers.
2. Description of the Related Art
Subterranean barriers are generally used to restrict the movement of underground water for pollution prevention, civil construction, or groundwater management. Vertical barriers are commonly made by slurry trenching, sheet piles, jet grouting, pressure grouting, and many other methods. Methods vary in depth capability, hydraulic quality, and the types of earth that can be subjected to the containment process.
There are many methods of constructing vertical barriers but few proven means of constructing a horizontal barrier without first removing the soil over the area where the barrier is needed. As removing the overburden soil may be costly or hazardous, construction of a horizontal barrier in situ may be desirable. Many landfills containing trash, municipal waste, and mining waste materials were developed with no liner at all and represent a potential threat to groundwater that could be remedied by construction of a bottom barrier. There are many earthen dams and levees, which are at risk of failure due to small leaks, that would benefit from a safe and inexpensive method of forming a flexible but water-tight vertical barrier down their centerline.
As described in U.S. Pat. No. 5,890,840, which is hereby incorporated by reference herein, a method of creating horizontal basin shaped barriers under a contaminated site has been contemplated. Horizontal directionally-drilled holes were drilled under the site and a pipe with several non-crossed cables running the length of the pipe was installed into each hole. At the edge of the site, where the pipes and cables exit the holes, one cable from each adjacent hole was selected and joined to the cable from the adjacent hole. The free end of these two cables at the other side of the site was attached to dozers, winches, or other pulling means to pull on the cables causing them to slice through the soil between the two holes. Dense fluid grout was continually supplied to the holes to fill the cut, e.g., swath or path, formed by the passing of the cable. The pipe served the purpose of orienting the cables and preventing rotation of the cables as they were initially pulled into the hole which would cause them to become crossed. Crossed cables would interfere with the cutting process.
Problems with this method included trying to keep the cables from crossing when drawing the pipe and cables into the hole and the tendency of the cables stretched along a curving borehole to cut into the walls of the holes such that the barrier did not follow the original path of the holes. The vertical curvature of the holes and the cable tension required to cut the path between adjacent holes would result in the cable cutting upward from the hole for a short distance before turning horizontally toward the adjacent hole. This vertical portion of the cut would not be expanded by the buoyancy of the dense fluid grout and so would be a significant defect in the otherwise uniform bottom barrier.
FIGS. 1a and 1b show a prior art process for forming a thin vertical subterranean hydraulic barrier. FIG. 1a illustrates the construction of thin diaphragm walls, or “panels” by jet grouting. In this method, cement grout is sprayed from jet nozzles 1 as a pipe 7 is moved upward through the ground which impinges the soil to form a mixture of cement grout and soil. In the centerline cross sectional view of the wall in FIG. 1b, the jet blast 2 from the nozzles 1 is directed in an “X” shaped pattern with an included angle 3 selected to help assure continuity of the wall. The pipe 7 is typically driven down into the ground to a desired depth using larger jet nozzles 4 on the tip of the pipe 7 that are pointed downward. After the pipe 7 reaches depth, a ball 5 is dropped to plug the larger jets 4 so that grout flows out of the smaller jets 1 that will create the jetted wall or barrier. 6. Intersection of the grouted soil cement panels depends on the pipes being properly aligned and the power and rate of movement of the jets 1 being suitable to completely cut through the soil between adjacent pipes.
In commercial applications, thin vertical or horizontal subterranean barriers may be constructed by using drill pipe 7 with 2 or 4 opposed orifices 1, “jets” or “nozzles,” that eject streams of fluid cement grout in opposing directions while raising the drill pipe 7 without rotation. When using two jets 1 on each side of the pipe 7, the jets 1 are each directed a few degrees, 10 to 45 degrees to either side of the direction of the adjacent drill pipe positions, to improve the chances of the spray from at least one intersecting the spray from the next pipe. Each stream of grout cuts vertical planar paths through the soil leaving a mixture of cementitious grout and soil that hardens into planar vertical panels. Multiple adjacent panels may be constructed such that they overlap to form a hydraulic barrier wall in situ in the ground.
These barriers are often called “X panel walls” 2 when made with 4 jets as in FIGS. 1a and 1b or “thin diaphragm walls” when made with only 2 jets. Such walls require much less time and material to form compared to jet grouted walls made of joined circular columns. However these thin walls are more likely to have leaks due to rocks, hard soil, or obstructions within the native soil that disrupt the penetration of the jet. Adjacent panels may also fail to intersect because of incorrect drill pipe orientation or variations in spacing between holes formed by the drill pipe. Sometimes the jets do not penetrate as far through the soil as expected or they are not oriented properly and miss the adjacent panel. These problems generally increase with increasing depth.
Even when formed as planned, these thin walls made of soil and cement sometimes do not work very well for several reasons. The permeability of jet grouted soil-grout mixture is relatively high. So, a thin wall does not impede water movement as much as a thicker wall made of interconnected columns. Also, such thin walls may crack due to soil movements and drying shrinkage. Traditional cement or cement and bentonite slurries often have lumps which partially plug a jet without the knowledge of the operator causing a defect in the wall.
Other installation problems exist. The jetting is generally only performed on the way out of the ground. Jetting with cement slurry typically forms panels up to 2 feet away from the drill pipe but adding a concentric jet of air around the jet can increase penetration up to 7 feet from the drill pipe allowing a 14 foot wide panel to be formed while returning large volumes of soil, water, and grout to the surface. Also, jet-grouted columns may be formed with molten wax using jet nozzles on a rotating drill pipe. One problem with this process is that the wax is far more costly than cement grout and thus the relatively large volume required to form jet grouted columns makes the use of molten wax too expensive for widespread use outside the nuclear industry.
Therefore, an economical, effective method and apparatus to form a barrier in a subterranean formation is needed.